If you gently push down on the opposite tensioner, the tape will tighten up and probably track OK, but this is no solution. Try "TAPEHEADS.COM", and get a username and password (always a pain, but).
there is a bunch of articles on all of the PITA things that go wrong with Teacs and how they need belts, calibration, tension adjustments, new pinch wheels, etc.
If you want it to work like new again, i know an expert repair service, although
he takes a long time to get the machine fixed. but it will be "as new" when he is done with it. OR If you're especially brave, you might have the skill and patience
to try a few things yourself.
they suck when they mess up, but when they work perfectly again, they are
wonderful contraptions and sound way better than most people realize.